1. Field of the Invention
The present invention relates to small-birefringent optical devices which are useful, for example, as optical disk substrates for recording and reproducing information optically, lenses, prisms, etc.
2. Description of the Prior Arts
Recently there is noticed the system of recording and reproducing high-density information, e.g. images and/or sounds, by using laser beams, wherein the information is recorded in the form of fine projections or depressions on disk substrates or in the form of changes in an optical property of recording films formed on disk substrates.
Disk substrates for use in the above recording and reproducing system are required to be transparent and in addition have such properties as good dimensional stability, high optical homogeneity, and small birefringence.
A plenty of duplicated substrates can be economically produced by using a resin material as a material of the disk substrate. However, it is widely recognized that the resin flows during the molding of the disk substrate and molecules are oriented during cooling so that birefringence is induced. This is a fatal defect for optical disk substrates.
The molecular orientation caused during molding is inevitable particularly in injection molding. From these point of view, a polymer comprising methyl methacrylate is the only resin material with small optical anisotropy for molding of disk substrates.
Known polymer comprising methyl methacrylate, however, have a drawback in that because of their high hygroscopicity, substrates formed thereof are inferior in dimensional stability, that is, the substrate warps and/or twists in high humid atmosphere.
This drawback is as described in detail, for example, in Nikkei Electronics (p. 133, issued Jun. 7, 1982). Therefore, aromatic polycarbonate resins having low hygroscopicity are in use as materials for acoustic compact disks.
On the other hand, it is difficult to decrease the birefringence of the molded substrates formed of aromatic carbonate resins since they contain aromatic rings, which have large anisotropy, in the backbone chain. While decreasing of the molecular weight and conditions of the molding are under investigation. However, since birefringence of the substrate originates from the material itself, it is impossible to constantly produce substrate with small birefringence and it is extremely difficult at present to produce small-birefringent substrates larger in diameter than acoustic compact disk by injection molding.
To improve dimensional stability which is a drawback of the polymer comprising methyl methacrylate, copolymers comprising methyl methacrylate and aromatic vinyl monomer are proposed [e.g. Japanese Patent Application (Laid-Open) Nos. 33446/82, 162135/82, and 88843/83]. However, these copolymer having aromatic rings cannot be practical use on account of their liability to large birefringence. For the disk substrates which is used not only for reproducing but also recording of information, a material is required to have further small birefringence and dimensional stability, but no resin has yet been found out that can enough satisfy these requirements.
With respect to other optical devices including lenses and prims, for which methacrylic resins and the like have been used as materials, there are needs for optical devices formed from a resin which is further small birefringence and superior in heat resistance, mechanical strength, and dimensional stability.
U.S. Pat. No. 4,373,065 discloses an optical recording device comprising an optically isotropic composition prepared by mixing two resins which have opposite optical anisotropies but are completely miscible with each other, in such a ratio that their optical anisotropies will be just compensated with each other to reduce the birefringence to substantial zero.
This patent further describes mixtures prepared by using a polyphenylene ether and polystyrene as component polymers having opposite optical anisotropies and showed that a film prepared such a composition of the mixtures that these anisotropies are just compensated with each other does not cause birefringence even when the film is stretched under the stress.
However, this patent does not reveal that an optical device such as an optical disk substrate, when formed from the above polymer composition by injection molding, exhibits small birefringence.
The present inventors have found that the birefringence of moldings such as optical disk substrates is not always small when these moldings are formed by injection molding, which is recently the most common method of forming optical devices, even from a mixture of two polymers which have opposite optical anisotropies in the solid state but are completely miscible with each other, wherein the mixing ratio is such that the optical anisotropies in the solid state are just compensated with each other.
That is, it has been found that optical materials such as an optical disk substrate having small birefringence can not be obtained by injection molding of any composition of polymer mixtures that is chosen by considering merely the optical anisotropy of each polymer in the solid state.
In recent years, attemps are in progress to produce plastic substrates of magneto-optical disks where the erasing and rewritting of information are possible.
Information recorded on such an optical disk of a magneto-optical type is read by focussing a polarlized laser beam through a lens on the recording medium of the disk and detecting a Kerr effect-induced slight rotation of the polarization plane of the laser beam reflected from the recording media. Accordingly, it is necessary in this case to use an optical disk substrate which scarcely causes the birefringence for oblique incident beams as well.
In addition, the optical disk substrate is required the high heat resistance since the substrate during information writting is heated by a laser beam.